Balls

Balls.js

let goldSilver = 0;
let goldGold = 0;

Array.prototype.random = function () {
  return this.splice(Math.floor((Math.random()*this.length)), 1)[0];
}

for(let i = 0; i < 1000000; i++) {
	let boxes = [["Gold", "Gold"], ["Gold", "Silver"], ["Silver", "Silver"]];
	
	let randomBox = boxes.random();
	let firstBall = randomBox.random();
	if(firstBall !== "Gold") continue;

	let secondBall = randomBox.random();
	if(secondBall === "Gold") goldGold++;
	else goldSilver++;
}

console.log("Gold -> Gold: ", goldGold)
console.log("Gold -> Silver: ", goldSilver)
console.log("Probability: ", Math.round(goldGold / (goldGold + goldSilver) * 100) + "%")